License maintenance: GPLv2 -> GPLv2+
Summary:
This is using the existing approvals for such changes listed on
https://techbase.kde.org/Projects/KDE_Relicensing.
With this the mailimporter repository should be clear of GPLv2 content
(not considering test code).
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D17133